2.2: Exploring Limits
What are some processes of the biosphere that move carbon in the Earth system?
Carbon moves among oceans, atmosphere, and land. The carbon cycle is part of a record of past climates on Earth, but it also acts on periods of days to years. In Lesson 2.1, you started investigating how carbon moves and changes forms in the Earth system. You looked at satellite data that showed carbon in the atmosphere and how it changes month to month. You also considered organisms with carbon, such as plants and the air from your lungs.
But there is another part of Earth that stores a huge amount of carbon: the ocean. It was in your box diagram in Lesson 2.1. In this lesson, you will focus on the oceans. This will help you better understand how carbon cycles among the land, atmosphere, and oceans. In Lesson 2.2, you will learn that:
Use the focus question to help to guide your thinking about how carbon moves in the Earth system.
In Unit 1, you investigated carbon moving into and out of water. When carbon moves into water, it becomes part of the aquatic ecosystems. Those ecosystems could be marine or freshwater. Your class will set up a model of an aquatic ecosystem and monitor carbon for one to two weeks. And as you learn more about the carbon cycle and climate, you will see how that model represents carbon in the Earth system.
Your model uses algae in containers. Your teacher will show you the setup. You will start with a culture of algae, fertilizer (that is, a "growth medium"), and some light. Your goal is to gather evidence that carbon is moving into the system.
As you started exploring in Unit 1, carbon moves among oceans, atmosphere, and land. In some settings, carbon moves quickly—over periods of days or years. But what are some of the main processes in the biosphere that affect how carbon moves in the Earth system? Two vital processes involve organisms.
Photosynthesis is the process where plants use carbon dioxide (CO2), water (H2O), and sunlight to make glucose (C6H12O6) and oxygen (O2). It also moves carbon from the atmosphere to the biosphere. Only some organisms are able to use light ("photo‐") to make, or synthesize, their cells and tissues, however. These organisms are called primary producers. These include plants, algae, and some bacteria.
Primary producers are the base of the food web. All other organisms rely on primary producers. For example, a horse is not a primary producer, but it does need to eat, or consume, grass. Thus, the horse is a consumer.
All the organic molecules, whether in an individual organism or an entire ecosystem, are called biomass. The diagram at the beginning of this lesson shows biomass for part of Earth.
6CO2 + 6H2O + E → C6H12O6 + 6O2
carbon dioxide + water + energy from the sun → glucose + oxygen
A pine tree is a good way to show the importance of this equation. It starts from a seed, but where does the mass come from for a seed to grow into a tree? Remember in our last lesson that we discovered that the mass comes from CO2 in the air. Though it does not seem like CO2 gas can produce a large tree, it does. After making glucose, the plant uses the glucose to make the cells and tissues of bark, wood, roots, and needles. It may seem hard to believe that organisms of the biosphere rely on carbon gas in the air, but they do. Whether cells and tissues are from producers or consumers, nearly all the biomass in ecosystems originates from CO2 in the air. Watch the time-lapse Radish Seed video again of a seed growing into a plant using only a wet paper towel. This is photosynthesis in fast-motion.
In Lesson 2.1, question 5b asked you where the mass of a growing plant came from. Check in your notebook for how you first answered the question. Revise your answer now based on what you are learning about photosynthesis.
But the carbon may not stay in organic molecules for long. What if the organism is using energy and burning calories to stay alive? Even organisms such as trees, plants, and algae have to use energy to stay alive. Or, if the organism dies, where does that carbon go? You have probably seen plants or foods that are rotten and decaying. Decomposers include the bacteria, fungi, worms, or insects that cause rot and decay. Decomposers get their energy from organic matter. You saw this in the Decomposition video about leaf and wood decomposition on the forest floor. In fact, nearly all organisms get their energy this way. The process is called "cellular respiration" because it happens in the cells of organisms. For an organism breathing oxygen (O2), its "food" can be represented as the sugar glucose (C6H12O6). Cellular respiration is the process of using oxygen and glucose to supply energy for the organism. This process also releases CO2 and water. The respiration reaction can be represented as:
C6H12O6 + 6O2 → 6CO2 + 6H2O + E
glucose + oxygen → carbon dioxide + water + energy for the organism
For some organisms, it might not appear that they have energy needs. Consider a tree or plant, for example. It does not expend energy like you do when you walk, run, or move your arms. However, it does require a lot of energy to grow the different tissues in limbs, leaves, roots, or flowers. Imagine a tree leafing out in the spring. This growth spurt is similar to the energy you use to grow. Some types of plants have poisons or molecules that keep consumers from eating their leaves. Making those extra kinds of molecules requires extra energy. That energy comes from the breakdown of organic molecules (such as glucose) during cellular respiration.
Get with a partner to answer the following questions about photosynthesis and respiration.
Perhaps your class cannot do the algae model. To see the results of another class of students who did the algae model, view the images at the bottom.
In the carbon cycle, carbon moves at different rates. For an ecosystem or region, this depends on whether photosynthesis is faster than respiration (P>R), or whether respiration is faster than photosynthesis (R<P).
You can explore real data to find out if photosynthesis or respiration is faster. These data were collected by high school students. The students had learned that oxygen was produced in photosynthesis. They put a large number of aquatic plants in an aquarium. They put the aquarium near a window in a room with no other lights. Using probes, they measured the dissolved oxygen and pH for two days. The student data is in the following graph. With a partner, explore the graph of the student data.
|Time||Rate of Photosynthesis (P) versus Respiration (R)||Oxygen (O2) in Water||Carbon Dioxide (CO2) in Water||pH of Water|
Now, let's go global! You can explore whether the physical processes in the aquarium or your algae model are also reflected in three key parts of the global biosphere. You will look at a few sources of data below for land, oceans, and atmosphere. Complete these steps with a partner.
Complete Lesson 2.2 with the questions below.
In this unit, you are studying how to track carbon as it moves in the Earth system. Knowing how carbon cycles today will help you make informed decisions in the future. It also helps us understand changes to the amount of carbon in the atmosphere in recent times. Understanding changes in the amount of carbon is important in thinking about future climate.
Rate-Limiting Factors on Photosynthesis
South Atlantic Ocean
You have seen how patterns of light affect the rates of photosynthesis and respiration. This can be day or night patterns, or seasonal patterns of light. You can investigate another factor that might limit productivity in the oceans: nitrates. Rate-limiting factors are factors such as light or nitrates that affect the total productivity in an ecosystem.
Nitrates contain the essential element nitrogen. Plants and all living things need nitrogen to grow. You can provide nitrogen to plants with fertilizer. You may have learned that nitrogen is a part of DNA and RNA. Nitrates and other nutrients are also often found in cold, deep waters of the ocean. Along coastlines, winds can move surface waters away from the shore. Colder, deeper waters move up to replace the surface waters. This process is called upwelling. Upwelling brings nitrates and other nutrients to the photic zone. The nutrients are vital to productivity in marine ecosystems. A lack of nitrates can limit the growth of primary producers.
Investigate with a partner whether light or nitrates limit productivity in the southern Atlantic.
If you are unable to see the interactive, click here to open it in a new tab.
Other Rate-Limiting Factors in Oceans
So far, you have seen that light can be a limiting factor for productivity on the land and in the oceans. If productivity is limited, carbon moves more slowly from the atmosphere to the ocean or land. What other limiting factors might also play a role?
You can select one of three areas to explore. For each area, you will be able to view the concentration of chlorophyll for each month across an entire year. You can compare this with three other variables that might affect the amount of chlorophyll.
Nitrate: Nitrates contain the essential element nitrogen. Plants, algae, and all living things need nitrogen to grow. A lack of nitrogen can limit the growth of producers. Scientists can use data from NASA satellites to calculate the concentration of nitrates. You added some growth medium to your algae. A main chemical of the growth medium is nitrate.
Light: NASA satellites measure the amount of sunlight across the globe. Plants and algae use light in the visible range (400 to 700 nm) for photosynthesis.
Aerosols: Aerosols are small solid or liquid particles suspended in air. A higher amount of aerosols may indicate a large amount of dust or smoke from a fire. Dust carries many important nutrients for plants and algae, including iron. So aerosols are a proxy for measuring the amount of iron. Iron may limit the growth of plants or algae.
These satellite images show a dust storm in western Africa. Particulates are swept from the Sahara Desert to the Atlantic Ocean. Nutrients in the desert dust serve as a "fertilizer" for the marine food web in the Atlantic Ocean.
The next RLF Viewer has three other areas to investigate productivity and carbon movement in marine ecosystems. Select a region to investigate with your partner.